Mystara Planes 3E
by RipvanwormerThe 3rd edition take on the types of planes and so on is explained here:
http://www.d20srd.org/indexes/planes.htm
By those definitions, I think Nightmare works quite well as a transitive plane (a place beings from distant planes can travel through to reach Mystara, and a place that touches both Outer Planes and the Prime, as the Nightmare Dimension did in the Immortals set).
I think Aelos, Old Alphatia, Laterre, and other similar worlds all qualify more or less as Material Planes - they're essentially Earth like, with the standard natural laws.
I think the Dreamlands and Spirit World don't qualify as Material Planes, since their laws of nature may differ extremely. Whether or not they're the same place, I think they'd work well as transitive planes coexistent with the Material Plane and perhaps some Outer Planes. It's possible that each Material Plane has its own Dreamlands and Spirit World, or at least its own Spirit World.
Limbo is, by definition, an Outer Plane. Whether or not it exists in the same place as the other Outer Planes is another matter - it could exist pretty much anywhere. It seems like among the Inner Planes opposite the Dreamlands is as good a place as any.
The other Outer Planes are either "Outer Planes" or demiplanes, depending on whether or not immortal beings have colonised them. Although there's something to be said for pocket planes utilised by wizards and so on not being the same kind of thing or even in the same place as the planes the immortals use.
Something like this, I think:
With the understanding that the Prime includes all the Material Planes.
I think I've actually made Limbo a transitive plane, here - it's the plane where souls transition from the Material Plane to the Outer Planes. Apparently you have to travel through the Spirit World to reach it; I didn't realise that was the case until I made the diagram, but it makes sense.
Spirit World, Faerie, the Ethereal, and Nightmare are all transitive planes coterminous and coexistent with the Material Plane, though not with one another. The Elemental Planes are coterminous and coexistent with the Ethereal Plane, and coterminous with the Material Plane through "wormholes" or elemental vortices. The Astral Plane is coterminous with the Ethereal and (many of) the Outer Planes. Some Outer Planes are coterminous only with other Outer Planes.
The Vortex Plane exists outside all the others, though it is coterminous with the Nightmare Plane and/or the Astral.